This invention relates to the production of a matted transparent paper which is improved in writability, paper strength, dimensional stability and secondary processability.
A tracing paper, a master copy paper and a paper for plotter are usually required to have a uniform transparency and a good writability with a pencil or an aqueous ink. Those papers have been generally prepared by subjecting papers made of heavily beaten pulp having a Canadian Standard Freeness (hereinafter referred to as CSF) of within the range of 50 to 100 cc or less to a treatment with heat and pressure under a high moisture content with the use of a calender. However, the heavy beating of pulp promotes an injury and a hydration of pulp fibers and the papers made of such heavily beaten pulp have various practical disadvantages such as a low paper strength for example tear resistance and a low dimensional stability to moisture. In addition, a paper sheet treated by for example a heated super calender for the purpose of obtaining a high transparency has a very smooth surface, and accordingly is inferior in a writability.
Japanese Patent Publication No. 5,326 of 1969 discloses a process for making a transparent paper in which natural pulp, synthetic pulp or a mixture thereof is formed into a paper and then heat and pressure are applied to the resultant paper under the condition of the moisture content of the paper being 10 to 30% with the use of a flat plate or a roll having such an unenven surface that its surface roughness, H max, which is defined in Japanese Industrial Standard (hereinafter referred to as JIS) BO601-1955; is within the range of 2 to 20 microns, and a height of the fine relief peak on the surface is within the range of 0.2 to 1 micron. This technique has improved a writability of paper which has never been satisfactorily obtained in the conventional calendering techniques by forming finely uneven structures on the surface of the paper with use of a particular calender system provided with the uneven surface roll. The paper thus obtained has an improved writability, but this technique still depends on the conventional heavily beaten pulp for obtaining a transparency which is one of the other important required characteristics. Since this technique depends on a limited method such as a sand blast method for forming the finely uneven structures on the surface of the roll, the processed surface of the roll is limited to the extremely fine structures having the patterns peculiar to the sand blast method. Therefore, a satisfactory and uniform transparency cannot be obtained when the paper is made of pulp having a CSF larger than 100 cc though it can be obtained in the case of the paper being composed of heavily beaten pulp. The above technique uses, practically, the pulp whose beating degree is within the range of 90 to 95SR according to a Schopper-Riegler method described in JIS P8121. This value of beating degree corresponds to a CSF of one figure value which shows that the pulp is beaten to an extremely high extent. The disadvantages due to such a heavy beating have been already described hereinbefore.
As described above, according to the prior arts, it has never been possible to obtain a tracing paper or a master copy paper superior in a uniformity of transparency, paper strength, writability, dimensional stability and secondary processability with the use of such an ordinarily beaten pulp that is used in making a general wood free paper.
According to the invention, the above described problems in the prior arts can be solved. This invention is based on a discovery which is against the heretofore accepted theory that a heavy beating is required for obtaining a sufficient transparency of sheet. This invention is characterized by that a paper is prepared from ordinarily or lightly beaten pulp and then heat and pressure are applied to the resultant paper with aid of a special embossing metal roll whereby the problem of decrease in transparency inherent in the use of such lightly beaten pulp is solved.
An object of the invention is to provide a matted paper which has a uniform transparency and is improved in writability, paper strength, dimensional stability and secondary processability.
Other objects and advantages of the invention will become apparent from the following descriptions.